JOP # 4
DIAGNOSTIC OF ERUPTING PROMINENCES AS ONSETS OF CMEs
Jean-Claude Vial, 9 May 1995, revised 9 September 1995
Objective:
To study the eruption of a (limb) prominence as the onset of a
coronal mass ejection
Scientific case:
The point of studying the onset of CME has been made in the JOP 3
document authored by Harrison et al. and is still a motivation for the
proposed program. But our objective is more focused here on the
investigation of the conditions of the eruption of a prominence often
associated with the CME (statistically more than 75%).
Of special interet here, is the determination of the physical conditions
in the EP (densities, ionization degree and especially temperature,
badly known up to now).
From ground-based observations, one will try to derive the magnetic
conditions which may be necessary and/or sufficient for the eruption.
On SOHO, one will get as much information as possible on the
thermodynamic state of the plasma, before, during and after the eruption.
All coronal instruments are observing together.
Method:
The method is described in JOP3.
In this program, we stress the importance of a reliable diagnostic of the
plasma in a very large temperature range since prominences are certainly
made of cool material (about 7000 K) which is (partially?) heated to
coronal temperatures (106K). CDS and SUMER cover such a
temperature range but the large field of view (fov) combined with a not
negligible temporal resolution makes the observation of faint to moderate
lines somewhat difficult. This is the reason why, with SUMER, we
propose to observe the FULL profile of the Lalpha line which offers a
powerful diagnostic tool, provided one uses the results of Non-LTE
transfer modeling. Moreover, this diagnostic will complement the
diagnostic performed at larger heights with UVCS.
Basically, our SUMER program differs from JOP3 because we want to
retrieve the full Lalpha profile.
Pointing and Target Selection: See JOP3.
Operating Details:
CDS, EIT, LASCO, UVCS programs are unchanged.
SUMER: The method is different from the one proposed in JOP 3 in
different aspects:
1/ choice of lines: we propose to drop the N V (1238) and Fe XII (1242)
lines because the Fe XII line is very faint and could noticeably increase
only for a compact flare-like event. We choose instead the S X (1196) line
which provides the higher temperature limit (5 105 K). In this way, we
access other lines such as N I (1200) and Si II (1194.5).
The lines selection is as follows:
O V (1218.4), La (1215.6), Si III (1206.5), N I (1200), S X (1196), Si II
(1194.5).
2/data extraction:
Contrary to JOP 3, we want 'full' line profile information.
For optically thin lines, we propose to compute the 2 moments of the line
profiles: integrated and Doppler shift. These parameters are necessary
for a comprehensive diagnostic of the plasma, including its motions.
For the optically thick Lalpha it is necessary to keep the wealth of
information contained in a profile which may be complex because of a
combination of temperature, density, and velocity variations.
3/ although the fov is rather large (120*300 arcsec2), it may be necessary
to follow the expanding structure, so a 'flag' initiated by the detection
of an apparent motion should be available and result in a move of the
pointing. This flag could be initiated by SUMER but also by CDS and EIT
which have a larger fov.
Operational Sequence
Initial pointing: selected limb position
Slit 1x120 arcsec^2
Scan Area 120*300 arcsec^2
Step Size 0.76 arcsec; 5 steps over 2s
Resulting Number of Scan Locations : 400 elementary steps
80 exposures
Dwell Time 2 s
Duration of Scan 160 s
Number of Scans preplanned: 100; after an event occurs: 100
Number of Scan Mirror Settings 1
Repointing: none except if FLAG
Total Duration: 4 hours and a half (if no event)
Line Selection Ly alpha (1216.7A) (on the bare MCP); O V
(1218), Si III (1206.5), N I (1200), S X (1196), Si II (1194.5)
Bins Accross Line: 25 pixels (Ly alpha), 2 'pixels' for other lines
Estimated Reduction Factor
* Selection 1x25x24 (Ly alpha); 5*2*24 (other 5 lines)
* Compression: for all lines, average over 5 spatial pixels; 2
moments computation for all lines except Ly alpha
* Reduction
Co-operation Requirements: all SOHO coronal instruments (see the
CME program JOP 3)
Notes (for Sumer):
The raster is made in the 'smear step' mode.
For data extraction, a mixture of different formats is required; a special
UDP is being written
The flag criteria (apparent motion in the field-of-view) and the reaction
to the flag (repointing) must be worked out.